In general, diesel engines are used as engines for hydraulic excavators included in the hydraulic work machinery. In the diesel engine, speed control is performed. In the speed control, when an actually detected engine rotational speed (hereinafter referred to as “actual engine rotational speed”) becomes lower than a target engine rotational speed with an increase in engine load, a fuel injection quantity is controlled so as to cause the actual engine rotational speed to approach the target engine rotational speed.
In hydraulic excavators, a variable displacement hydraulic pump is driven by the diesel engine to drive a hydraulic actuator, such as an arm cylinder, with discharge oil of the variable displacement hydraulic pump. Therefore, when pump discharge pressure increases with operation of the hydraulic actuator, the engine load increases and the actual engine rotational speed decreases. When the actual engine rotational speed decreases in this manner, the above-described speed control is performed. The speed control has a delay in response to a decrease in the actual engine rotational speed, thereby causing, a phenomenon in which the actual engine rotational speed decreases during a response time period, that is, lug-down of the engine. The quicker the operation of the hydraulic actuator from a stopped state, in other words, the more sudden an increase in pump absorption torque, the more likely the lug-down of the engine is to increase.
In the past, lug-down of the engine has been suppressed by controlling the pump absorption torque at the time when an operating lever device, serving as operation instruction means for giving an instruction to cause the hydraulic actuator to operate, is non-operated, and the pump absorption torque at the time when the operating lever device is operated (see Patent Literatures 1 and 2).